Oil is found in subterranean formations or reservoirs in which it has accumulated, and recovery is initially accomplished by pumping or permitting the oil to flow to the surface of the earth through wells drilled into the oil-bearing stratum. Oil can be recovered from such producing zones only if certain conditions exist. There must be adequate permeability or interconnected flow channels through the pore network of the oil-bearing stratums or "pay zone" to permit the flow of fluids therethrough and recovery efficiency (RE).
In the primary oil recovery stage, the RE is influenced by the natural energy or drive mechanisms present, such as water drive, gas cap drive, gravity, drainage, liquid expansion, relative permeability of reservoir formation, and combinations thereof within the formation and this natural energy is utilized to recover petroleum. In this primary phase of oil recovery, the oil reservoir natural energy drives the oil through the pore network toward the producing wells. When the natural energy source is depleted or in the instance of those formations which do not originally contain sufficient natural energy to permit primary recovery operations, some form of supplemental or artificial drive energy must be added to the reservoir to continue RE. Supplemental recovery of enhanced recovery is frequently referred to as secondary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment. Enhanced recovery usually encompasses waterflooding or gas injection with or without additives, and other processes involving fluid or energy injection whether for secondary or tertiary oil recovery such as the use of steam or heated water.
Secondary recovery is a term utilized to mean any enhanced recovery first undertaken in any particular underground formation. Usually it follows primary recovery but can be conducted concurrently therewith to expedite production. Waterflooding is the most common method of secondary recovery.
Tertiary recovery refers to any enhanced recovery undertaken following secondary recovery. Broadly, tertiary recovery encompasses such procedures as miscible displacement, thermal recovery, or chemical flooding.
All of these procedures have been and, as noted, are being utilized to try to recover as much oil as possible from any given formation, but none is completely satisfactory. Many are expensive procedures not only in terms of equipment to be able to enhance the recovery, but also in terms of the chemicals and techniques utilized.
Perhaps most importantly it has been found that in many cases the particular technique used is extremely limited in terms of type of oil reservoir in which the recovery technique can be utilized and that a broad procedure for universal use has not been found.
This is particularly true with respect to waterflooding; probably the most inexpensive and widely practiced enhanced recovery technique. Water does not displace oil with high efficiency since water and oil are immiscible and the interfacial tension between water and oil is quite high. Accordingly, waterflood has produced incremental oil recovery amounting to about 10 to 15% of the original oil in place (OOIP) in the reservoir. In efforts to increase the amount of oil displaced from the formation and bring it to the surface, efforts have been made to utilize certain chemicals, mostly surfactants, to decrease the interfacial tension (IFT) between the injection water and the reservoir oil in order to displace and trap the oil in the underground formation and bring it to the surface. Such technique is referred to as surfactant flooding.
However, problems have occurred with such chemicals either because they are not sufficiently active to adequately displace the oil or are costly. More importantly, their effectiveness is limited by the reservoir heterogeneity, various reservoir fluids, high salinity, high bivalent ion concentration, high temperature, and continuous changes in such conditions along the pore channels in the reservoir. The chemicals tend to be unstable in or to be decomposed by such conditions and they suffer chromatographic changes.
No satisfactory stable displacement material or technique has been found which is economic, effective in the presence of highly concentrated brine, high temperatures, and/or hardness of the reservoir water, or other reservoir conditions.